This invention relates to the field of communication systems, and in particular to packets that are transmitted using a spread-spectrum code.
Spread-Spectrum techniques are used to modulate an information signal such that the modulated signal appears as noise. The information is modulated by a pseudo-random sequence of bits, and can be demodulated, or despread, by the same pseudo-random sequence. This modulation is commonly referred to as Direct-Sequence Spread Spectrum (DSSS). The modulated signal is spread across a bandwidth that is substantially larger than the bandwidth of the information signal, and has the apparent effect of increasing the noise-floor of receivers that receive this signal. Applying the same pseudo-random sequence to the modulated signal allows the information signal to be detected within this apparent noise.
A significant characteristic of a proper pseudo-random spread spectrum code is that a coherent output is produced only when the decoding code sequence is applied substantially in phase with the encoding code sequence. If the received signal is decoded with a code-phase that is out of phase with the transmitter, and the code is a proper pseudo-noise code having defined uniqueness characteristics, such as a maximum length shift register sequence, then the decoding of this out-of-phase signal produces a noise output. U.S. Pat. No. 5,537,397, “SPREAD ALOHA CDMA DATA COMMUNICATIONS”, issued Jul. 16, 1996, to Norman Abramson, and incorporated by reference herein, discloses a technique that uses this phase-dependency characteristic to allow multiple transmitters to use the same code concurrently. Each active transmitter is assigned a particular code-phase by the receiving and/or controlling node, and the timing of the transmitter is controlled (advanced/retarded) by the receiver to assure that each transmitter's signal is received at a code phase that is distinguishable from other transmitters.
Because each transmission from the multiple transmitters appears as noise to the receiving system, and because the transmitted signal is dependent upon the spreading code, the code-phase, and the value of the information bit being transmitted, the determination of any one of these three parameters generally depends upon knowledge of the other two parameters. Therefore, in a conventional spread-spectrum system that uses a known spreading code, a sequence of known information bits, commonly termed a message ‘preamble’, is transmitted to allow a receiver to determine the code phase of the transmission. Subsequent transmissions of information bits that are unknown to the receiver (i.e. new information from the transmitter) can then be decoded at the receiver, based on knowledge of the spreading code and this determined code phase of the transmitter.
The ‘cost’ or ‘overhead’ associated with transmitting a sequence of known information bits with each message or packet of actual information is dependent upon the relative sizes of the known information sequence and the actual information sequence. Typically, the known information sequence required for code-phase detection is in the order of tens of bits; if the actual information sequence is in the order of thousands of bits, the overhead cost of transmitting the preamble is fairly insignificant. If, on the other hand, the actual information sequence is in the order of a hundred bits, a relatively significant amount of transmission resources are consumed in sending the preamble. This overhead cost is of particular significance for mobile transmitters, as each transmission consumes battery power in proportion to the length of the transmission.
In conventional spread spectrum systems, the preamble to the message serves multiple purposes. In addition to facilitating the determination of the transmitter's code phase relative to the receiver, the preamble is typically also used to identify bit polarity, locate start and stop bits, align data packets, and so on. Because a preamble of known information bits is required for conventional spread spectrum code-phase determination, there is generally little or no additional cost or overhead associated with structuring such a preamble to efficiently perform these additional packet-synchronizing and message-decoding tasks.
Copending U.S. patent applications, Ser. No. 11/681,759, “DETECTION OF MULTIPLE-CODE-PHASE TRANSMISSIONS”, filed 3 Mar. 2007 for James F. Stafford, Scott A. McDermott, and William F. Seng, and Ser. No. 11/876,747 “COHERENT DETECTION WITHOUT TRANSMISSION PREAMBLE”, filed 22 Oct. 2007 for Scott A. McDermott, James F. Stafford, and Luis G. Jordan, each incorporated by reference herein, disclose techniques that facilitate determination of a transmitter's code phase relative to a receiver without requiring a known information bit sequence. Other techniques, such as disclosed in U.S. Pat. No. 6,985,512, “ASYNCHRONOUS SPREAD-SPECTRUM COMMUNICATIONS”, issued 10 Jan. 2006 to Scott A. McDermott and Leif Eric Aamot, and its CIP, U.S. patent application Ser. No. 10/208,882, “SPREAD-SPECTRUM RECEIVER WITH PROGRESSIVE FOURIER TRANSFORM”, filed 31 Jul. 2002 for Scott A. McDermott, each also incorporated by reference herein, also allow for transmitter code-phase detection without requiring a known information bit sequence.
The elimination of the need for a known information bit sequence to determine a spread-spectrum transmitter's code phase, however, does not eliminate the need for a transmission preamble, per se. The aforementioned packet and message synchronization and decoding tasks that are generally placed on the message preamble are still required, even though the code-phase detection task may have been eliminated.
It would be advantageous to eliminate the need for a preamble of known bits in a spread-spectrum communication system. It would also be advantageous to provide a defined structure or message architecture that allows for efficient packet and message synchronization and/or decoding without reliance upon a preamble of known bits.
These advantages, and others, can be realized by using existing message fields and/or message parameters to perform the packet and message synchronization and decoding tasks that are conventionally performed by message preambles that use a known bit sequence, thereby eliminating the need for message preambles. In example embodiments, the unique identifier of each transmitter is structured to facilitate determination of bit polarity and the start of each packet; packet sequence numbers use an unconventional counting sequence to assure synchronizing bit transitions; and so on. Other techniques, such as the use of run-length limited (RLL) message encoding to assure within-packet bit transitions, are also used to enhance clock synchronization.
Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions. The drawings are included for illustrative purposes and are not intended to limit the scope of the invention.